The colloid inside thyroid follicles is primarily composed of which protein?

• A. Thyroglobulin
• B. Albumin
• C. Immunoglobulin
• D. Casein

Answer: (A)

Thyroid colloid is mainly thyroglobulin, a large glycoprotein produced by the thyroid follicular cells and secreted into the follicular lumen to act as the storage substrate for hormone synthesis. Iodine is organified and tyrosine residues within thyroglobulin are iodinated and then coupled to form the thyroid hormones T3 and T4 inside this protein scaffold. When needed, thyroglobulin is endocytosed and proteolytically cleaved to release the active hormones. Other proteins listed, such as albumin, immunoglobulins, or casein, are not produced or stored in thyroid colloid and do not participate in thyroid hormone synthesis.